Placement of WITH(NOLOCK) in nested queries - sql

Question

In the Following query where would I place WITH(NOLOCK)?
SELECT *
FROM (SELECT *
FROM (SELECT *
FROM (SELECT *
FROM (SELECT *
FROM dbo.VBsplit(#mnemonicList, ',')) a) b
JOIN dct
ON dct.concept = b.concept
WHERE b.geo = dct.geo) c
JOIN dct_rel z
ON c.db_int = z.db_int) d
JOIN rel_d y
ON y.rel_id = d.rel_id
WHERE y.update_status = 0
GROUP BY y.rel_id,
d.concept,
d.geo_rfa

Answer 1

You should not put NOLOCK anywhere in that query. If you are trying to prevent readers from blocking writers, a much better alternative is READ COMMITTED SNAPSHOT. Of course, you should read about this, just like you should read about NOLOCK before blindly throwing it into your queries:
Is the NOLOCK SQL Server hint bad practice?
Is NOLOCK always bad?
What risks are there if we enable read committed snapshot in SQL Server?
Also, since you're using SQL Server 2008, you should probably replace your VBSplit() function with a table-valued parameter - this will be much more efficient than splitting up a string, even if the function is baked in CLR as implied.
First, create a table type that can hold appropriate strings. I'm going to assume the list is guaranteed to be unique and no individual mnemonic word can be > 900 characters.
CREATE TYPE dbo.Strings AS TABLE(Word NVARCHAR(900) PRIMARY KEY);
Now, you can create a procedure that takes a parameter of this type, and which sets the isolation level of your choosing in one location:
CREATE PROCEDURE dbo.Whatever
#Strings dbo.Strings READONLY
AS
BEGIN
SET NOCOUNT ON;
SET TRANSACTION ISOLATION LEVEL --<choose wisely>;
SELECT -- please list your columns here instead of *
FROM #Strings AS s
INNER JOIN dbo.dct -- please always use proper schema prefix
ON dct.concept = s.Word
...
END
GO
Now you can simply pass a collection (such as a DataTable) in from your app, be it C# or whatever, and not have to assemble or deconstruct a messy comma-separated list at all.

Answer 2

Since the question really is, "where should I put NOLOCK". I am not going do debate the use of OR reformat the query with better joins. I will just answer the question.
In no way am I intending to say this is the better way or to say that the other answers are bad. The other answer solve the actual problem. I'm just intending to show where exactly to place the lock hints as the question asks
SELECT *
FROM (SELECT *
FROM (SELECT *
FROM (SELECT *
FROM (SELECT *
FROM dbo.VBsplit(#mnemonicList, ',')) a) b
JOIN dct WITH (NOLOCK) -- <---
ON dct.concept = b.concept
WHERE b.geo = dct.geo) c
JOIN dct_rel z WITH (NOLOCK) -- <---
ON c.db_int = z.db_int) d
JOIN rel_d y WITH (NOLOCK) -- <---
ON y.rel_id = d.rel_id
WHERE y.update_status = 0
GROUP BY y.rel_id,
d.concept,
d.geo_rfa

Answer 3

Like this, to use the tidiest method.
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
SELECT * FROM (SELECT * FROM
(SELECT * FROM (SELECT * FROM
(SELECT * FROM dbo.VBsplit(#mnemonicList,',')) a ) b
JOIN dct ON dct.concept = b.concept WHERE b.geo = dct_variable.geo_rfa) c
JOIN dct_rel z ON c.db_int = z.db_int) d
JOIN rel_d y ON y.rel_id = d.rel_id
WHERE y.update_status = 0
GROUP BY y.rel_id,d.concept,d.geo_rfa
SET TRANSACTION ISOLATION LEVEL READ COMMITTED
However, unless you are using this for reporting purposes on an active database, enabling dirty reads may not be the best way to go.
Edited as (NOLOCK) itself is not deprecated except as described here: http://technet.microsoft.com/en-us/library/ms143729.aspx.